The Keeping Quality of Whole Milk Powder. II. The Effect of Preheat Temperatures on the Development of Peroxides and Ferricyanide-Reducing Groups under Different Storage Conditions

Part I (3) of this study gave the flavor and residual lipase activity of milk powders made with different preheat treatments and stored air-packed at 45 and 85 ° F. and nitrogen-packed at 85 ° F. This paper reports the development of peroxide and potassium ferricyanide-reducing groups as measures of the keeping quality of these same powders. REVIEW OF LITERATUREPeroxide formation.There is general agreement that the first product formed in the oxidation of an unsaturated fat is a peroxide.However, the breakdown products formed when the peroxide is split at the double bond, and not the peroxides themselves, are responsible for the unpleasant oxidative rancidity (tallowy) flavors and odors.This theory holds true for butterfat according to Holm (10).He found taUowiness to occur in butterfat when the peroxide values reach 1.20 (m.e. of oxygen per kg. of fat) and are broken down by heating; however, values of 0.80 before destruction leave the butterfat still edible.Powiek (15) believes the off-flavors and odors of oxidized fats are due primarily to medium molecular weight aldehydes (7 and 9 carbon) and the lower molecular weight aldehydes, acids, ketones, etc., do not play an important part.The amount of fat decomposed in the oxidation process may be not more than 0.1 per cent, according to Pritzer and Jungkunz (16).Peroxide formation in whole milk powders.The use of the peroxide test to predict the onset or degree of oxidized flavor in whole milk powders is subject to differences of opinion by different authors.Hollender and Tracy (9), using the peroxide test of Smith ( 19), conclude that taste is superior to the peroxide test as a means of detecting oxidized flavor at an early stage.Pyenson et al. (18), using the peroxide test of Chapman and McFarlane (2), analyzed 180 samples of spray-dried whole milk powder and reported that peroxide values are not a satisfactory criterion of keeping quality or palatability.Findlay et al. (6) using the test of Chapman and MeFarlane (2), found that peroxide development was correlated with flavor scores when high preheat temperatures were used (190-200 ° F. for 20 sec.) and the lowest peroxide values were obtained at these high preheat temperatures.Greenbank